Orthodontic brackets that are directly bonded to a patient's teeth, are now common. These brackets each typically include an archwire portion adapted to receive a straightwire and ligature ties to impart corrective force to the teeth and an integrally formed base pad portion. The base pad portion is formed opposite to the archwire portion and includes a tooth abutting surface shaped to conform to the shape of the wearer's teeth. This tooth abutting surface is adapted to be attached to the teeth using a non-toxic orthodontic adhesive.
Experience has shown that a common problem of such orthodontic brackets is the propensity of the base pad to break away from the tooth after bonding has occurred, and the skill required by the orthodontist to position the bracket relative to a wearer's tooth to impart corrective force without having portions of the bracket being brought into interfering contact with the wearer's gum, mouth etc. As to the former, cleaning of the wearer's teeth using a common tooth brush and cleaning paste, gel, powder and the like, for example, can erode the adhesive between the tooth and pad. In such weakened condition, the pad can be loosen from the tooth during normal masticatory activities. As to the latter situation, experience has also shown that when the gingival and occlusal tips of the L-shaped wing members are constructed in a rhomboidal configuration for alignment purposes, a series of sharp parameter edges are generated which can be a hazard to the wearer.
One approach to increase the bonding strength is described in U.S. Pat. No. 4,369,033 for "ORTHODONTIC BRACKET ASSEMBLY", Webb et al in which a series of cavities are formed in the tooth abutting surface of the pad during casting. While such cavities do increase the surface area of the pad and hence allow more adhesive (and surface area) adjacent to the surfaces to be bonded, the fact that the walls of the cavities must diverge outwardly at such surfaces, creates a mechanically weak joint, especially when twisting moments of force are applied to the bracket during masticatory activities including normal brushing. In such instances, such moment arms create large shear forces along longitudinal (as opposed to transverse) portions of the surfaces of the bracket to be bonded relative to the wearer's tooth.